As the Internet becomes increasingly driven by Web services and on-line databases, the simple keyword search interfaces provided by most Web search engines are no longer adequate for searching, navigating an exploring web services and on-line databases. When navigating or searching for Web services, a user is often more interested in Web services from a particular domain, or Web services containing a particular set of input attributes, or Web services that are similar to a given example of a Web service. For example, a user may want to find all on-line bookstore databases, or all on-line databases with an input for ISBN and price, or all on-line databases similar to “amazon.com.” Current search engine technology does not support such queries.